Already picked 2 large bunches of alsters so far this year. a bit late flowering but they were going up until Xmas, so I reckon they need a bit of a rest!!
plant them deep and keep picking them, when the flowers are finishing pull the whole stem out of the ground and the resulting wound spurs it on to make more flowers.
Lots of compost and manure and they will form a huge clump and 3 years later you will be dividing them and have loads - I have them every where!!! All from 2 3" pots 5 years ago
One of the best value plants you can get, nearly always get 6 months flowering out of them.